Does changing your Compute Device, change all computes of Blender?
Only rendering process can use GPU acceleration, specifically rendering with Cycles Render Engine alone; even switching to Blender internal ignores all benefits of GPU acceleration.

CUDA acceleration is not supported anywhere else in Blender. The only remaining parts, other than the OpenGL Viewport and UI Drawing, that use GPU acceleration (like Open Subdivide evaluation and Compositor OpenCL compute), are unaffected by the CUDA computing device settings, and use their own different settings in specific places.